Liquid vacuum pumps of this type are employed, for example to degas extruders. When, possibly toxic gases are drawn in, it is also possible for contaminants, such as, for example melt particles, etc., to be aspirated as well. Both the toxic components of the gases as well as the contaminants are largely absorbed by the compression liquid. In order to prevent these absorbed and possibly coarser contamination particles from causing any damage to the liquid ring vacuum pump, a known approach is to clean the compression liquid in a closed circulation path that includes a replaceable filter insert, for example a candle filter. The disadvantage here is that the closed circulation path must be interrupted to replace the candle filter whenever the candle filter has become clogged after an extended period of use.
Another approach is known in principle where finer contaminants, such as for example toxic components upstream of the pump, are filtered out by precondensation units. These contaminants are intended to condense out here. They are deposited in the precondensation unit and must be disposed of from time to time. However, the contaminants condense only at a significant cost, in particular, in terms of cooling capacity, due to the high temperatures of the plastic melt gases, with the result that the known precondensation units function in an unsatisfactory and inadequate fashion.
As a result, two filter units are required if both coarse as well as fine particles are to be collected. In order to clean or replace the filter inserts, at least the compression-liquid closed circulation path must be interrupted, and possibly the entire liquid ring vacuum pump must be stopped. In order to clean the precondensation unit, typically two expensive condensers must be provided, so one of the condensers can operate while the other condenser is undergoing elaborate cleaning, for example by solvents, and its waste must be transferred to the hazardous waste area.
DE 296 15 006 discloses a circulation system for water ring pumps in which coarse contaminants are separated from the compression liquid along an inclined plane, while finer contaminants are filtered through a cylindrical, spherical, or conical filter. If, however, the inclined plane and/or the filter gets clogged, the closed closed circulation path must be opened to enable cleaning, and the contaminants then removed. Continuous operation is thus just as impossible as is the filtering of ultrafine contaminants.
DE 41 18 787 discloses a process-integrated working-fluid cleaning system for compressors in which the working fluid of the compressor is cleaned by a pervaporation-membrane module. A membrane is employed here through which the contaminated working fluid can pass. After passing through the membrane, the contaminants evaporate on the back side of the membrane. This enables only evaporating contaminants to be extracted from the compression liquid. The membrane would clog up if there were solids in the compression liquid. The membrane would then have to be replaced. To do this, the liquid ring vacuum pump would also have to be stopped and the otherwise closed closed circulation path for the compression liquid would have to be opened.
If the goal is to degas polyester melts with toxic contaminants, relatively large quantities of water are drawn in aside from contaminants by the liquid ring vacuum pump. These quantities of water are transferred to the compression liquid. This water must be extracted from the closed circulation path of the compression liquid. Special care must be taken to ensure an especially thorough cleaning of the compression liquid so that this water also does not have to be disposed of as hazardous waste.